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Abstract

We propose a semi-infinite 1-D photonic crystal approach for designing artificial reflectors which aim to reproduce color changes with the angle of incidence found in biological periodic multilayer templates. We show that both the dominant reflected wavelength and the photonic bandgap can be predicted and that these predictions agree with exact calculations of reflectance spectra for a finite multilayer structure. In order to help the designer, the concept of spectral richness of angle-tuned color-selecting reflectors is introduced and color changes with angle are displayed in a chromaticity diagram. The usefulness of the photonic crystal approach is demonstrated by modelling a biological template (found in the cuticle of Chrysochora vittata beetle) and by designing a bio-inspired artificial reflector which reproduces the visual aspect of the template. The bio-inspired novel aspect of the design relies on the strong unbalance between the thicknesses of the two layers forming the unit cell.

Figures (8)

Reflectance spectra measured on a biological sample (cuticle of Chrysochroa vittata beetle - ventral side) at angles of incidence equal to 20° (magenta line), 30° (red line) and 45° (green line) using TE-polarized light. A metallic mirror was used as reference in a variable-angle specular reflectance measurement setup. Note the shift of the dominant reflected wavelength towards the shorter wavelengths as the angle of incidence was increased.

Evolution of the chromaticity coordinates as the angle of incidence is increased from 0° to 80° by steps of 5° in the case of a 20-periods SiO2/TiO2 multilayer structure (n1 = 1.5, n2 = 2.7, d1 = 194 nm, d2 = 10 nm).